Liquid crystal displays (LCDs) respond to the RMS value a square wave voltage signal applied to the LCD. The input voltage necessary to drive the LCD are selected according to the turn-on and turn-off voltages of the LCD material and the duty cycle of the square wave. Typically, five or six greater or lesser number of voltages could be used, depending on the duty cycle.
In the prior art, an integrated driving circuit is used to drive the LCDs. The five voltages are generated external to the driving circuit via a supply voltage and a resistor ladder which divides the supply voltage into the desired five voltages.
In many cases, the supply voltage is a battery, which has an output which will vary over the life of the battery. The output level of the supply voltage will affect the contrast of the LCD; hence, a potentiometer is often used to trim the generated voltages input to the driver circuit. Consequently, to maintain a constant contrast, the potentiometer would necessarily be adjusted throughout the life of the battery, which is inappropriate in many circumstances. Further, the contrast of the LCD will vary with temperature, which also complicates the goal of a constant contrast.
Therefore, a need has arisen in the industry for a circuit which is operable to drive an LCD at a substantially constant contrast over a normal operating temperature range and over the life of a battery. Further, it is desirable that the entire LCD driver circuit be integrated such that the driving voltages are generated on the integrated circuit, in order to simplify design.